Technique for controlling current limiter of motor

[0002] This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from my application APPARATUS AND METHOD OF CONTROLLING CURRENT LIMITER OF MOTOR filed with the Korean Industrial Property Office on Sep. 12, 1998 and there duly assigned Serial No. 37670/1998.

[0003] 1. Field of the Invention

[0004] The present invention relates to a motor controlling technique, and more particularly, to a technique for controlling a current limiter of a motor for limiting current supplied to the motor by controlling the current limiter according to the rotational speed of the motor so that accurate control of the motor according to the type of execution mode is possible.

[0005] 2. Description of the Related Art

[0006] In general, a motor controlling apparatus uses a current limiter to protect the motor from overcurrent. The current limiter limits current supplied to the motor according to a voltage supplied to a control port. That is, the current limiter cuts off the current supplied to the motor when a voltage under a threshold value is supplied to the control port. When a voltage over the threshold value is supplied, the current limiter increases the current supplied to the motor in proportion to the level of the supplied voltage.

[0007] According to conventional technology, since a uniform voltage level is supplied to the control port of the current limiter, the current supplied to the motor is limited to a particular value regardless of the type of execution mode.

[0008] In a VCR, a slow playback mode, which is one of a group of special playback modes, is for displaying an image more slowly than a normal playback mode, which is accomplished by periodically repeating a still mode and the normal playback mode. When the slow playback mode is executed by repeating the normal playback mode and the still mode, control of a brake in the motor is needed. Control of the brake is generally performed by supplying a reverse voltage to the motor for a predetermined time to reduce the speed of the motor so that a tape runs for a predetermined time and the motor comes to a stop. In this state, since the motor stops out of a target position according to a load, characteristic picture motion in a screen is not displayed smoothly.

[0009] To solve the above problem, it is an object of the present invention to provide a technique for controlling a current limiter of a motor by controlling the amount of current which is limited by the current limiter according to the rotational speed of the motor according to a playback mode so that accurate control of the motor is possible.

[0010] Accordingly, to achieve the above object, there is provided an apparatus for controlling a current limiter of a motor for protecting the motor by limiting the amount of electric current supplied to the motor and controlling a driving force of the motor, which comprises: a frequency detector for detecting the rotational frequency of the motor; a limiter controller for generating a current limiter control signal corresponding to the rotational frequency detected by the frequency detector; and a current limiter for limiting the amount of current supplied to the motor in accordance with the current limiter control signal.

[0011] To achieve another aspect of the object of the present invention, there is provided a method of controlling a current limiter of a motor for protecting the motor by limiting the amount of current supplied to the motor according to a current limiter control signal and controlling the driving force of the motor, which is achieved by (A) setting a duty cycle value D₀of a pulse width modulation signal which is the current limiter control signal for each mode, (B) determining whether the present execution mode corresponds to a mode for altering a limit amount of current of the current limiter, (C) detecting a rotational frequency f_cof the motor, and (D) if it is determined in step (B) that the present execution mode is the mode for altering the limit amount of current, altering D₀set in step (A) according to the value of f_c.

[0012] A more complete appreciation of the invention, and may of the attendant advantages, thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:

[0013] FIG. 1 is a block diagram showing the structure of a capstan motor servo of a VCR adopting a current limiter according to the present invention;

[0014] FIG. 2 is a flowchart for explaining a method of controlling a current limiter of a motor according to the present invention;

[0015] FIG. 3 is a flowchart for explaining the control of a slow playback mode in the VCR adopting the current limiter of the present invention;

[0016] FIG. 4 is a flowchart for explaining a method of controlling a current limiter in the brake voltage application step in FIG. 3; and

[0017] FIG. 5 is a timing diagram of major signals related to FIG. 3.

[0018] FIG. 1 shows the structure of a capstan servo block of a VCR adopting a current limiter of the present invention. Referring to the drawing, the capstan servo block includes an oscillator 101, frequency division circuits 102, 104, and 108, a control head 103, a phase controller 105, a capstan motor 106, a C.FG (capstan frequency generator) signal generator 107, a speed controller 109, a driving amplifier 110, a system controller 111, a frequency detector 112, a limiter controller 113, a low pass filter 114, and a current limiter 115.

[0019] The C.FG signal generator 107 included in the capstan motor 106 generates a signal (a C.FG signal) having a frequency corresponding to the rotational speed of the capstan motor. The speed control of the capstan motor in a normal playback mode is executed using the C.FG signal.

[0020] The method of controlling the speed of the capstan motor is described as follows.

[0021] The C.FG signal is converted to a rectangular waveform signal by the frequency division circuit 108. The speed controller 109 delays the C.FG rectangular waveform signal and generates a trapezoidal pulse maintaining a predetermined timing relation with the C.FG signal using the delayed C.FG rectangular waveform signal. A voltage proportional to the pulse width of the C.FG rectangular waveform signal is sampled from the trapezoidal pulse and, by holding the sampled voltage and outputting the same to the driving amplifier 110, the speed of the capstan motor 106 is controlled.

[0022] The method of controlling the phase of the capstan motor in a normal playback mode is described as follows.

[0023] An oscillation signal output from the oscillator 101 is divided by the frequency division circuit 102 and input to the phase controller 105. A control signal detected by the control head 103 is divided by the frequency division circuit 104 and input to the phase controller 105.

[0024] The divided oscillation signal is used not only for the phase control of the capstan motor 106 but also the phase control of a drum motor. Thus, the divided oscillation signal uniformly maintains the relative position of a tape driven by the capstan motor and a video head.

[0025] In the phase controller 105, a trapezoidal pulse is generated from a rising (or falling) edge of the divided oscillation signal and the control signal is divided to generate a sampling pulse. Then, the trapezoidal pulse is sampled according to the sampling pulse and, by holding the sampled voltage, an error voltage which is a differential voltage from a reference voltage is output to the speed controller 109.

[0026] In the speed controller 109, the control of speed is performed by using the sampled and held voltage generated from a capstan motor speed control loop. Also, the phase of the capstan motor 106 is controlled by adding the error voltage generated from the phase controller 105 to the voltage for speed control. The error voltage output from the speed controller 109 is converted by the driving amplifier 110 to a motor driving voltage and current and applied to the current limiter 115.

[0027] In the frequency detector 112, the frequency of the C.FG signal output from the C.FG signal generator 107 is detected and output to the limiter controller 113. Then, in the limiter controller 113, a pulse width modulation (PWM) type current limiter control signal, in which the duty cycle of a pulse is altered according to the frequency of the C.FG signal, is generated by a control function set according to an execution mode. The PWM type current limiter control signal is rectified by the low pass filter 114 and is output to the current limiter 115.

[0028] Accordingly, the current limiter 115 receiving the driving voltage output from the driving amplifier 110 limits the amount of current supplied to the capstan motor 106 according to the rectified limiter control signal. That is, the operation of the motor 106 can be accurately controlled by limiting the amount of current supplied to the motor 106 by the current limiter 115 according to the rotational speed of the motor 106.

[0029] Referring to the flowchart of FIG. 2, a method of controlling the current limiter of a motor according to the present invention will now be described.

[0030] In step 201, the duty cycle value D₀of the PWM type current limiter control signal is set for each mode. In step 202, the present execution mode is determined. In step 203, the rotational frequency f_cof the motor 106 is detected. For example, the rotational frequency f_cof a motor is detected by determining the frequency of the C.FG signal detected by a magnetic device attached to the motor. In step 204, a PWM type current limiter control signal, in which the duty cycle of a pulse is altered according to the frequency of the C.FG signal, is generated by a control function set according to the execution mode.

[0031] Next, an example of a method of controlling the current limiter of a motor in a forward direction slow playback mode of a VCR will be described in detail.

[0032] FIG. 3 is a flowchart for explaining the control of a forward direction slow playback mode in the VCR.

[0033] In step 301, a speed multiple (1/N) in the slow playback mode and the number (C₁and C₂) of the C.FG signal for moving a section for one field are initially set. Here, C₁is the number of C.FG pulses occurring during ST1 (see the timing diagram (e) of FIG. 5), which is the section from a point when a positive control pulse is generated to a point when a capstan brake voltage is applied, to determine the length of one field execution section in a normal playback mode constituting the slow playback mode. C₂is the number of C.FG pulses occurring during ST2 (see the timing diagram (e) of FIG. 5) which is the section from a point when a negative control pulse is generated to a point when a capstan braking voltage is applied.

[0034] In step 302, it is determined whether the present execution mode is the forward direction slow playback mode. In step 303, if it is determined t hat the present mode is the forward direction slow playback mode in step 302, a still mode constituting the slow playback mode is executed. In step 304, while the still mode is executed, the number of rotations T of a drum are counted and it is determined whether T is (N−1)/2. Here, a half (½) rotation of the drum corresponds to one field, and the 1/N speed slow playback mode as in the present invention is constituted by one field playback mode and (N−1) field still mode.

[0035] In step 305, when it is determined in step 304 that the still mode is executed such that T is (N−1)/2, the capstan motor 106 is driven to execute the playback mode for one field. In step 306, control of the speed and phase is performed, after the capstan motor 106 is driven for a period K1 shown in the timing diagram(c) of FIG. 5 instep 305. Instep 307, it is determined whether a control pulse, as in the timing diagram (e) of FIG. 5, is generated while the one field playback mode constituting the slow playback mode is executed. In step 308, if it is determined in step 307 that a control pulse is generated, a counter is operated to count the number of C.FG pulses. In step 309, it is determined whether the control pulse detected in step 307 is positive. In step 310, if it is determined that the detected control pulse is positive in step 309, it is determined whether the value C of the counter is equal to the number C₁set in step 301. In step 311, if the control pulse detected in step 309 is negative, it is determined whether the value C of the counter is equal to the number C₂set in step 301.

[0036] In step 312, when it is determined that the value C of the counter is equal to the number C₁or C₂in step 310 or step 311, respectively, as shown in timing diagrams (d) and (e) of FIG. 5, a braking voltage corresponding to a reverse voltage is supplied to stop the capstan motor 106. This is because the value of the counter corresponds to the execution of one field playback mode. In the control of the brake, strong braking is initially applied for a period B1 and weak braking is applied for a period B2. As shown in timing diagrams (h) and (i) of FIG. 5, the weak braking is applied for a second period B2 in which the phases of the C.FG1 pulse and the C.FG2 pulse change by alternating the phases of the C.FG1 pulse and the C.FG2 pulse in the forward direction and the reverse direction. Then, reverse compensation is executed for a period B3 as shown in the timing diagram (d) of FIG. 5.

[0037] After applying the capstan braking voltage to the capstan motor 106 in step 312, the system returns to step 302 and repeats from step 302 to step 312.

[0038] Step 303 and step 312 of the above steps are for applying braking voltage to a motor. A method of accurately controlling the motor by controlling the current limiter of the motor according to the present invention will be described with reference to FIG. 4.

[0039] In step 401, a PWM type duty cycle value D₀, which is a control signal of the current limiter, and a proportional constant K₀for changing the duty according to the rotational speed of the motor in the slow playback mode, are set.

[0040] In step 402, it is determined whether it is a braking application period T_BRKin the slow playback mode, which can be determined by using a forward direction/reverse direction control signal (CAP F/R) of the motor shown in the timing diagram of FIG. 5.

[0041] In step 403, if it is determined in step 402 to be the braking application period T_BRK, a rotational frequency of the motor, i.e., the frequency of the C.FG signal f_cis detected.

[0042] In step 404, a current limit value of the motor is altered by changing the duty cycle of the PWM type current limiter control signal according to the frequency f_cof the C.FG signal. That is, the pulse duty cycle of the current limiter control signal is altered in the braking application section in proportion to the rotational speed of the motor. A limit current waveform by the current limiter, altered according to the rotational speed of the motor, is shown in the timing diagram (k) of FIG. 5.

[0043] In step 405, if it is determined in step 402 not to correspond to the braking application period T_BRK, the pulse duty cycle of the current limiter control signal is determined to be the D₀initially set in step 401 to control the current limiter.

[0044] According to the above braking control, the motor can be controlled to accurately stop at a desired position by limiting not only the braking application voltage but also the amount of current supplied to the motor, considering the rotational speed of the motor, in the braking application section.

[0045] According to the above operation, by altering the control voltage of the current limiter of the motor according to the rotational speed of the motor, effects due to the load condition of a deck, an axial load of the motor, or deviation in torque can be minimized so that the motor can be stopped at a desired position.

[0046] As described above, according to the present invention, the current limiter of the motor is controlled by a pulse-width modulated signal in which the duty cycle changes, i.e., a PWM signal, according to rotational speed of the motor so that a load condition or deviation of an axial load of the motor is minimized to accurately control the motor.

[0047] It should be understood that the present invention is not limited to the particular embodiment disclosed herein as the best mode contemplated for carrying out the present invention, but rather that the present invention is not limited to the specific embodiments described in this specification except as defined in the appended claims.